Abstract : The objective of this work, performed in the frame of the Industrial Chair in Bioplastics, financed by Mines ParisTech and Arkema, l'Oreal, Nestle, PSA and Schneider Electric, is to provide a systematic study of the relationships between the compounding conditions and the structure of biocomposites based on polypropylene/ flax and Tencel® fibres. In particular, the behaviour and the rupture of fibres were studied in detail during melt processing in an internal mixer and a twin screw extruder.The fibres were observed in situ during shear flow in a matrix by rheo-optics. The decohesion of flax bundles was shown to be made easier for fibres with higher initial aspect ratio. The fibres fragmentation occured by fatigue and is caused by an accumulation of strain and mechanical energy. At the breaking point, flax and Tencel® fibres are tearing and fibrillating, whereas elementary flax fibres break at “kink bands”. The analysis of fibres size distributions after compounding has corroborated rheo-optical observations. When processing conditions are severe, each “kink band” becomes a breaking point, and the final fibres length is equal to the mean length between two “kind bands”. The short flax bundles dissociate and break up less after compounding as compared to long bundles. As a result, the rheological properties of composites are different. The initial size of Tencel® fibres has almost no effect on fibre final dimensions, provided that they are not too long and thus do not make agglomerates. The mixing time seems to be decisive to preserve fibres aspect ratio. The cumulative strain was shown to be a better parameter than specific mechanical energy to describe fibres rupture for both Tencel® and flax fibres. Uniaxial tensile properties were characterized and correlated to the processing conditions and to final dimensions of fibres.